There are many types of vibrato, I dunno if someone here knows
Maite Kelly, she was also doing it very often (she was even trying not to do it and now her singing is much better but when live you can still spot it) but her vibrato was like a machine gun, very fast almost spinning, I really hated that.
What Ayu does is definately on higher level and doesn't strike that much but I still think she recently overuses it.